A process and an apparatus for the production of a plastic article having high-precision surface structures and/or high-luster surfaces is known from the present applicants' DE 41 08 236 C1. That known apparatus involves a not inconsiderable level of machine expenditure, and that has an effect on the production costs of the plastic articles produced with that apparatus or by means of that process. The specified plastic articles are for example Fresnel lenses of a highly transparent plastic material. The plastic material used is preferably PMMA.
A process and an apparatus of the kind set forth in the opening part of this specification are known from the German journal: "Kunststoffe" 80 (1990), issue 5, pages 583 to 587.
Therein the reduction of the mold cavity from the pre-molding cavity to the definitive mold cavity which corresponds to the plastic article to be produced is effected by a linear movement of the mold portions relative to each other in such a way that the mold cavity between the top mold portion and the bottom mold portion diminishes in parallel relationship. In that situation the viscous plastic material introduced into the pre-molding cavity is uniformly distributed from the centrally disposed inlet on all sides, that is to say in all directions, perpendicularly to the injection direction. That requires a certain amount of force and energy to be applied, as well as involving a considerable level of expenditure in terms of machinery or apparatus.
The object of the present invention is to provide a process and an apparatus of the kind set forth in the opening part of this specification, wherein it is possible, at a comparatively low level of machine expenditure, inexpensively to produce plastic articles of large area and small wall thickness, enjoying good optical properties.